The present invention relates to an improvement in a bandpass filter using dielectric resonators which shows a desirable propagation characteristic.
A prior art dielectric resonators bandpass filter (DR-BPF) comprises a plurality of dielectric resonators arranged in an array within a metal housing, metal screws associated in one-to-one correspondence with the dielectric resonators to adjust their resonant frequencies, and an input and an output connectors mounted on the housing. Two probes extend into the housing one from the input connector and the other from the output connector such that the connectors respectively are electromagnetically coupled with those dielectric resonators which are located at both ends of the array.
Basically, the role which a bandpass filter is to fulfill is transmitting signals which lie in a desired frequency band while intercepting all the frequencies outside the desired band. However, the prior art DF-FPF involves propagation of some needless or spurious modes one of which is the higher-order mode resonance of the dielectric resonators. Another spurious mode is the resonance of the metal housing or, more precisely, resonance due to interaction of the metal housing and the dielectric resonators which are installed in the housing. Stated another way, while an input signal is transmitted through the DR-BPF to be outputted therefrom, the spurious higher order mode of each dielectric resonator and the spurious mode due to resonance of the housing are propagated together with a desired dominant mode. In this manner, since the prior art DR-BPF is not furnished with a function of implementing a frequency characteristic which suppresses propagation of spurious modes, all the spurious modes are allowed to propagate through the DR-BPF. Propagation of those spurious modes is problematic in realtion to a DR-BPF which is applied to a communications system and others which are in practical use.